Servo Adjustment
6.4.2 Basic Rules of Gain Adjustment
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Generally speaking, the responsiveness of the position loop cannot be higher than that of the
speed loop. Therefore, to increase the position loop gain, you must first increase the speed
loop gain. If only the position loop gain is increased, oscillation will result in the speed refer-
ence and positioning time will increase, not decrease.
Positionloopgaincanbeincreasedonlytothepoint whereoscillation beginsin themechani-
cal system.
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If the position loop response is faster than the speed loop response, speed reference output
from the position loop cannot follow the position loop response due to the slow speed loop
response. Therefore, smooth linear acceleration or deceleration will not be possible and the
position loop will keep accumulating errors, thus increasing the amount of speed reference
output.
As a result, the motor speed will be excessive and the position loop will try decreasing the
amountofspeedreference output.The speedloop responsivenesswill bepoor, however,and
the motor will not be able to catch up with the speed reference. As a result, the speed refer-
ence will oscillate as shown in the following graph. If this happens, reduce the position loop
gain or increase the speed loop gain to prevent the speed reference from oscillating.
Speed
reference
Time
Actual speed reference output from controller.
Speed reference as a result of calculation in controller.
Figure 6.2
Speed Reference with Position Loop Gain and Speed Loop Responsive-
ness Not Well-balanced
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The position loop gain must not exceed the natural frequency of the mechanical system. For
example,ifthemechanicalsystemisanarticulatedrobot, therigidity ofthe machinerymech-
anism isvery lowbecause the mechanism incorporatesa wave reduction gear and the natural
frequency of the mechanical system is 10 to 20 Hz. In this case, the position loop gain can
be set to 10 to 20 (1/s).
If the mechanical system is a chip mounting machine, IC bonding machine, or high-preci-
sion machining tool, the natural frequency of the system is 70 Hz or more. Therefore, the
position loop gain can be set to 70 (1/s) or higher.
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When high responsiveness is required, it is not only important to ensure the responsiveness
of the servo system that is employed (the controller, Servopack, motor, and encoder), but it
is also necessary to ensure that the mechanical system have high rigidity.
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